A conventional incandescent lamp utilizes a filament of a refractory material, such as plain or doped tungsten which is electrically heated. When operated at and above the temperature at which the filament incandesces, it supplies visible wave length energy and energy in the infrared range. In a typical incandescent lamp, the infrared energy is radiated from the lamp and wasted as heat. The lamp filaments generally are of the helical coiled type, single coiled or coil-coiled, which are either mounted in a U-shaped arrangement or in an elongated horizontal or vertical mounting arrangement.
Incandescent lamps have been proposed which employ an infrared (IR) radiation reflective coating in combination with an optically shaped envelope to reflect IR energy back to the filament. The energy received by the filament raises its operating temperature and, therefore, decreases the amount of energy needed to heat the filament to its operating temperature. This results in a decrease in the total amount of power consumed by the lamp to produce the same amount of light output, thereby resulting in an energy saving.
In the design of a filament for a conventional incandescent lamp, reflected and returned infrared radiation plays no part in the design consideration. The design of such filaments for conventional lamps usually needs only the specification of parameters such as operating voltage, operating wattage, lumen per watt required, the operating temperature, and the desired operating life. From this, the resistance of the filament is calculated and the filament is constructed.
It has been found that conventional filaments, for example, those of the coiled coil-type for lamps of 100 watts or below, are unsuitable in an incandescent lamp which utilizes a radiation reflective coating. Such filaments have a relatively large length/diameter ratio (about 19:1 for a 100 watt filament) and, due to the extended length, there is a large temperature gradient between the central portion of the filament and its ends. This gradient limits the life of the filament to that characteristic of the hot central portion while simultaneously limiting the light output to a value characteristic of the lower, mean temperature value of the filament. The temperature gradient is greatly reduced in the non-IR reflecting lamp environment where it does not essentially alter the life-light output relationship.